Aromatic polyamides that remain soluble in their polymerization mixture have been known for a long time. Many such polymers containing meta-phenylene rings were synthesized in the 1950's {S. L. Kwolek and H. H. Yang, "History of Aramid Fibers" in "Manmade Fibers: Their Origin and Development" ed. by R. B. Seymour and R. S. Porter, Elsevier Applied Science (1993)}. The most notable of these polymers is poly(m-phenylene isophthalamide) (MPD-I). Because of its excellent thermal and textile-like properties, the fiber of MPD-I was commercialized first by Du Pont Co. in 1962 under the tradename of Nomex aramid, later by Teijin Ltd. of Japan under the tradename of Teijinconex, and by the former USSR under the name of Fenilon.
In the preparation of aromatic polyamides, the method of low temperature polycondensation is widely used which employs an amide solvent and often an alkali salt. The amide solvents for low temperature polycondensation include hexamethylphosphoramide (HMPA), N,N'-dimethyl acetamide (DMAc), N-methyl-2-pyrrolidone (NMP) and other derivatives. HMPA must be used with caution because of its suspected carcinogenic properties. DMAc and NMP are both used commercially. The alkali salts include calcium chloride, lithium chloride, lithium hydroxide, and the like. These salts are added to a polymerization system to improve the solubility of the polymer in the polymerization mixture, or to neutralize the polymerization mixture, so as to achieve a high degree of polymerization. Soluble aromatic polyamides such as MPD-I are often polymerized without alkali salts because of their high solubility in amide solvents.
Fibers of soluble aromatic polyamides can be prepared conveniently by directly spinning from the polymerization solution followed by processing. The spinning process includes the conventional dry spinning (U.S. Pat. Nos. 3,287,324 and 3,360,598) and wet spinning (U.S. Pat. Nos. 3,414,645, 3,642,706, 3,751,546, 3,869,429, 4,073,837, 4,342,715, 4,842,796) processes. The as-spun fibers are generally processed by washing, wet drawing, drying and hot drawing to achieve certain properties. The dry spinning process has the disadvantages of relatively high cost of solvent recovery and potential risk of environmental contamination. As a result, the wet spinning process has been favored over the dry spinning process in recent years.
In many cases of wet spinning, the polymer solution is spun from a spinneret into an aqueous coagulation bath containing at least 40% by weight of calcium chloride and at a temperature of 50.degree. C. or higher. Further, most polymer solutions contain calcium chloride from the solvent/salt system or from neutralization of the polymer solution. The high content of calcium chloride in the coagulation bath reduces the diffusion rate of calcium chloride from a spinning fiber. This results in a highly solvated fiber that will draw readily. The drawing is further facilitated by the use of a high coagulation bath temperature. However, the high temperature of the coagulation bath tends to accelerate the diffusion rate of calcium chloride. Such contradictory effects of high salt content and high temperature of coagulation and draw baths have heretofore been overlooked.